The medium answer: COM3 and above have no standard address or IRQ, hence
communications software (Kermit or anything else) can't always find them, in
which case you have to specify the address and IRQ, using a sequence like:

SET COM3 \x3e8 5
SET PORT COM3

(You need both commands, in the order shown.) You must also beware of
"missing" COM ports. To use (say) an internal modem as COM4 when there is no
COM3 is not straightforward.

The long answer: Read section 6 of the KERMIT.BWR file on your MS-DOS
Kermit 3.14 diskette (the version at kermit.columbia.edu in kermit/a, file
mskerm.bwr, might be newer).